Photomultiplier tubes are well known per se. Basically, a PMT is a tube which emits electrons when exposed to light or other radiation. A typical PMT includes a photocathode at one end of the tube, accelerating and focusing electrodes, a series of dynodes, and an anode. The photocathode at one end of the tube comprises a photoemissive material which ejects electrons in response to photons which hit the material. An associated power supply biases the accelerating and focusing electrodes more positively than the photocathode to accelerate the electrons away from the photoemissive material of the photocathode and axially through the tube. The power supply also biases the series of dynodes, one more positively than the last, to attract the accelerated electrons. As each electron hits an attracting dynode, the electron causes the dynode to eject one or more secondary electrons. The secondary electrons, in turn, are attracted to and hit the next dynode, ejecting still more electrons, creating a cascade effect. The cascade of electrons continue through the center of the tube toward the anode. The anode collects the electrons at the other end of the tube and produces a signal indicating the amount of light or other radiation to which the photoemissive material of the photocathode has been exposed. The multiplying effect of a PMT is evident from this example: a single electron hitting the first dynode can create a cascade of several thousand or even millions of secondary electrons at the anode. An example of a PMT is described in U.S. Pat. No. 4,639,638 to Purcell et al. for a Photomultiplier Dynode Coating Materials and Process, Issued Jan. 27, 1987.
FIG. 1a is a schematic diagram of a Photomultiplier Tube (PMT) 10 with one type of conventional power supply. The power supply 12 which biases the components of the PMT 10 can comprise an alternating current source 14 and a Cockcroft-Walton Circuit 16. This circuit 16 is known per se. The CW Circuit 16 comprises discrete elements, such as diodes 18 and capacitors 20, which are hard-wired in a ladder circuit. A first stage of the CW circuit 16 multiplies the voltage of the voltage source 14. Successive stages of the CW circuit 16 multiply the voltage of the preceding stage. Separate stages of the ladder comprising the CW Circuit 16 are connected to successive dynodes 22 of the PMT 10, for instance, to bias one dynode more positively than another in the direction of its anode 23. An example of a Cockcroft-Walton Circuit is described in U.S. Pat. No. 5,191,517 to Stephenson, for Electrostatic Particle Accelerator Having Linear Axial and Radial Fields, Issued Mar. 2, 1993, and assigned to the same assignee as this invention.
FIG. 1b is a schematic diagram of a Photomultiplier Tube (PMT) having conventional driving circuitry. In this case, the power supply comprises a voltage source 25 and a voltage dividing bleeder string 24. A bleeder string 24 comprising a series of resistors 26 connects to the PMT 10. Each resistor 26 of the bleeder string 24 is connected to bias an adjacent accelerator stage of the PMT 10.
Typically, the power supply, such as a CW Circuit 16, is assembled to the PMT 10 in a linear arrangement. Specifically, one end of the ladder comprising the CW Circuit 16 is secured to the anode-end of a PMT, such that longitudinal axes of the CW circuit 16 and of the PMT 10 are aligned. This arrangement results in a rather narrow PMT assembly. However, this arrangement is relatively long. The resulting length consumes valuable space in a well logging tool, for example.
Many companies provide photomultiplier tubes (PMTs) with miniature high voltage bases however they are not designed for the rigorous environmental demands of oil well-logging. This is because the high voltage power which drives the PMT is connected to the PMT using a multiholed socket and mate, or pin-and-header arrangement, which is fragile at best. Such high voltage devices available on the market utilize an end-to-end, linear Cockcroft-Walton (CW) ladder which delivers voltages to each stage of the PMT.
One application of a PMT is well-logging. Typically, the PMT and a radiation source are carded on a skid of a logging tool. High voltage power supplies, carded by the tool, connect to the PMT on the skid. The PMT's are mounted on the skid to be closer to the radiation source, which in turn produces better statistics and a faster log of the borehole. The power supplies are mounted in the tool itself to minimize skid length, which in turn reduces displacement of the skid off the mudwall of the borehole at deviations or wash-outs. After the logging tool is lowered into a borehole, the skid is extended from the tool to engage the mudwall of the borehole. The radiation source generates radiant energy into the earth formation surrounding the borehole, and the PMT detects returning radiation as the logging tool is winched up the borehole. The PMT produces an output in response to the detected radiation, which is interpreted to indicate characteristics of the earth formation.
The Micro-Channel Plate (MCP) detector has the same general purpose as the PMT, that is optical radiation detection. However, the MCP and PMT have different operating parameters and different applications. For example, the MCP is count-rate limited compared to the PMT, which is unacceptable in well-logging applications. The MCP needs only 3 high voltage outputs. One voltage is for the plates of the MCP. The plates supply a fairly hefty current, approximately 30 micro amps at 3-4 kV, typically using a 3 stage Cockcroft-Walton (CW) ladder to accomplish this. A second voltage biases the anode or phosphor of the MCP. If for an anode, only a few hundred volts are needed. If for a phosphor, a few thousand volts (sometimes 5 KV) may be needed usually with a 3 stage CW ladder. Typically, the power needed for this supply is small. A third voltage biases the cathode of the MCP. This voltage only has to provide a few hundred volts which are floating on top of the plate supply. Typically this supply is a simple rectifier. Even lower power is needed for the third supply than the second.